The invention relates to a double belt moulding machine for the production of foamed plastic plates in a continuous strand. The machine has two endless conveyor belts that rotate in opposite directions. These can for example be straps or steel strips, which are guided over a drive pulley, slideways serving as support elements and around a guide pulley. The conveyor belts can also be plate belts, which are guided over drive and guide sprocket wheels and which are equipped with support rollers, which can run along support elements in the form of guide rails. The two facing, parallel sections of the two belts running in the same direction define the two flat sides of the zone used for continuous foamed plastics strand moulding. The two narrow sides of this zone can be also bounded by circulating side belts or by fixed side rails, whose position is adjustable.
In the hitherto known constructions of such double belt moulding machines, the conveyor belts were driven by a common electric drive motor coupled with the belts by drive pulleys or sprocket wheels. A precise synchronism of the active belt sections defining the moulding zone could therefore not be obtained or could only be obtained by considerable expenditure on mechanical means. However, synchronism of the belt sections is very important, because the formation of shear forces in the foamed material strand, which could destroy its fine structure, must be avoided. It is difficult to fulfill this requirement, because synchronism of the active belt sections and uniformity of the tensile forces transferred by the belt sections to the material of the foamed plastic strand are dependent on varied and not readily controllable parameters. Thus, for example, slip can occur in the case of the friction-based transfer of torque from the drive pulleys to the straps or steel strips. In addition, varying divergences in the diameters of the drive pulleys can occur due to contamination or abrasion. The tensile forces can also be influenced by contamination of the support plates of the conveyor belts. Furthermore a stressing action is exerted by the weight of the belt sections defining the mould channel at the top and bottom and the weight of the materials introduced into the channel, particularly the foam material, together with its coatings and supports, on the lower belt section and its support plate, which leads to an increase on the tensile force in the lower belt. In addition, the force of the mixture foaming in the mould channel stresses both the lower and upper belt sections. Immediate compensation is necessary for all the effects disturbing the uniformity of the tensile forces in the active belt sections.
In addition, vertical adjustability of the upper belt is required to provide for the production of foamed plastic plates of different thicknesses. The provision of such a vertical adjustability is extremely difficult on the basis of purely mechanically adjustable coupling means between the drive motor and drive pulleys or the sprocket wheels of the belts.